Various films have been applied to windows to reduce glare and to obtain solar screening for an interior of a structure, such as a home, building or car. For example, a plastic film may be dyed to provide desired optical properties or may be coated with a number of layers to acquire the optical properties. A film that provides solar screening is one that has a low transmission in both the visible range (400 to 700 nm) and the near infrared range (700 to 2100 nm). To reduce glare, the transmission of visible light (T.sub.VIS) must be controlled.
Primarily through absorption, dyed films can be fabricated to provide a wide range of T.sub.VIS values. However, dyed films generally do not block near infrared solar energy and, consequently, are not completely effective as solar control films. Another shortcoming of dyed films is that they often fade with solar exposure. When the films are colored with multiple dyes, the dyes often fade at different rates, causing unwanted color changes over the life of the film.
Other known window films are fabricated using vacuum-deposited grey metals, such as stainless steel, inconel, monel, chrome or nichrome alloys. The deposited grey metal films offer about the same degrees of transmission in the visible and near infrared portions of the solar spectrum. As a result, the grey metal films are an improvement over dyed films with regard to solar control. The grey metal films are relatively stable when exposed to light, oxygen and moisture, and in those cases in which the transmission of the coatings increases due to oxidation, color changes are generally not detectable. After application to clear float glass, grey metals block light transmission by approximately equal amounts of solar reflection and solar absorption.
Vacuum-deposited layers such as silver, aluminum and copper control solar radiation primarily by reflection. Because of the high reflection in the visible spectrum (i.e., high R.sub.VIS), films having these vacuum-deposited layers are useful in only a limited number of applications. A modest degree of selectivity of transmission in the visible spectrum over transmission in the near infrared spectrum is afforded by certain reflective materials, such as copper and silver.
Traditionally, the best glare reducing coatings have been sputtered grey metals, such as stainless steel, chrome and nickel. The graph of FIG. 1 is a transmission spectrum 10 for a sputtered nichrome coating that is designed to transmit approximately 50% of the light at the center of the visible light spectrum (i.e., T.sub.VIS =50%). The nichrome is affixed to a 3.2 mm-thick plate of float glass. As can be seen, the transmission of energy is controlled in both the visible and near infrared portions of the solar spectrum. A slight degree of wavelength selectivity is observed due to the iron oxide in the glass.
In the graph of FIG. 2, the visible reflectivities of single and double layer nichrome films of various thicknesses are shown as a function of the corresponding visible light transmissions. (Here, double nichrome films refer to a construct in which two optically isolated sputtered coatings are employed, with the films being separated from each other by a relatively thick (22 micrometers) layer, such as a laminating adhesive.) While not shown in FIG. 2, the nichrome layer thicknesses decrease from left to right. As can be seen, the R.sub.VIS value decreases and the T.sub.VIS value increases as the nichrome layers become thinner. The comparison between the single and double layer nichrome films evidences that the double layer of nichrome has a substantially reduced R.sub.VIS value for the same T.sub.VIS value. For example, at a T.sub.VIS value of 20%, the single nichrome coating has an R.sub.VIS value of 24%, while the double nichrome coating has an R.sub.VIS value of 13%. As the nichrome layers become thinner, the R.sub.VIS values of the two films converge.
The percentages of solar rejection achieved by films with single and double layers of nichrome are compared in the graph of FIG. 3. Solar rejection is defined as: EQU solar rejection=solar reflection+(0.73.times.solar absorption).
Within the art, solar rejection is often calculated using solar energy distributions as given in the ASTM E 891 method. The slightly better solar rejection noted for the low transmission single nichrome coatings relative to the twin nichrome equivalents is due to solar reflection differences.
A low visible light transmission and low visible light reflection film utilizing double layers of nichrome is disclosed in U.S. Pat. No. 5,513,040 to Yang. The patent discloses a solar control film having two or more transparent substrates, each bearing a thin, transparent and discontinuous film of metal having low R.sub.VIS and a degree of visible light blocking capacity. The substrates are arranged and laminated into a composite, such that the visible light blocking capacities of the metal films are effectively combined to provide a composite having low visible light transmittance, i.e., a low T.sub.VIS. The discontinuous films of nichrome are attached using an adhesive layer.
The possibility of using metal nitride films in window-energy applications was discussed by C. Ribbing and A. Roos in an article entitled, "Transition Metal Nitride Films for Optical Applications," which was presented at SPIE's International Symposium on Optical Science, Engineering and Instrumentation, San Diego, July/August 1997. Single layers of TiN, ZrN and HfN were specifically identified. The article discusses the use of the materials in low emissivity coatings to replace noble metals, such as silver and gold. It is noted that the low emissivity coatings will not reach as high a selectivity as the current noble metal-based multi-layers, but may find use in aggressive environments, because of their excellent stability.
What is needed is a solar control member for application to a window or the like in order to achieve a high selectivity of visible transmission to near infrared transmission, with a controlled visible reflection and with age stability. What is further needed is a repeatable method of fabricating such a solar control member.